Display Device and Electronic Device

ABSTRACT

A dark portion in an image seen by a viewer is expressed more precisely, whereby the image given a greater sense of depth is displayed. A display device in which pixels each include a light-emitting module capable of emitting light having a spectral line half-width of less than or equal to 60 nm in a response time of less than or equal to 100 μs and are provided at a resolution of higher than or equal to 80 ppi; the NTSC ratio is higher than or equal to 80%; and the contrast ratio is higher than or equal to 500, is provided with a circuit converting an image signal having a given grayscale into an image signal capable of representing an image on the low luminance side by high-level grayscale.

TECHNICAL FIELD

The present invention relates to a display device. The present inventionrelates to an electronic device including a display device.

BACKGROUND ART

A display device that displays a stereoscopic image using a binocularparallax is known. Such a display device is configured to display, onone screen, an image to be seen from the position of the left eye of aviewer (an image for left eye) and an image to be seen from the positionof the right eye of the viewer (an image for right eye). The viewer seesthe image for left eye with the left eye and the image for right eyewith the right eye and is thus allowed to see a stereoscopic image.

For example, in a glasses system, an image for left eye and an image forright eye are alternately displayed on a screen of a display device insynchronization with a shutter provided in a pair of glasses, wherebythe left eye of a viewer is allowed to see only the image for left eyeand the right eye of the viewer is allowed to see only the image forright eye. Thus, the viewer can see a stereoscopic image.

Further, in a display device using a parallax barrier system whichallows a viewer to see a stereoscopic image with naked eyes, a screen isdivided into many regions (e.g., strip-like regions). The regions arealternately allocated to the region for right eye and the region forleft eye, and a parallax barrier is provided to overlap with theboundaries of the regions. In each the divided region, an image forright eye is displayed on the region for right eye, and an image forleft eye is displayed on the region for left eye. With the parallaxbarrier, the regions for displaying the image for right eye are hiddenfrom the left eye of a viewer and the regions for displaying the imagefor left eye are hidden from the right eye of the viewer; consequently,the left eye is allowed to see only the image for left eye and the righteye is allowed to see only the image for right eye. Thus, the viewer cansee a stereoscopic image with the naked eyes.

Note that a display device including a switchable parallax barrier forachieving switching between a flat image display mode and a stereoscopicimage display mode is known (Patent Document 1).

In addition, a light-emitting element in which a layer containing alight-emitting organic compound is provided between a pair of electrodesis known. This light-emitting element is a self-luminous type;therefore, high contrast and high speed of response to an input signalare achieved. A display device to which this light-emitting element isapplied is known (Patent Document 2).

REFERENCE Patent Document

-   [Patent Document 1] PCT International Publication No. WO2004/003630-   [Patent Document 2] Japanese Published Patent Application No.    2011-238908

DISCLOSURE OF INVENTION

In the case of using a display device that displays a stereoscopic imageusing a binocular parallax, a distance between a screen of the displaydevice and the left/right eye of a viewer is almost uniform regardlessof an image displayed. Therefore, in some cases, a distance between aviewer and a screen on which the left/right eye of the viewer is focusedis different from a distance, which provides a binocular parallax,between the viewer and the object in an image displayed on the screen.Thus, there has been a problem in that the difference has caused strainon the viewer.

The present invention is made in view of the foregoing technicalbackground. Therefore, an object of the present invention is to providea display device that can display an image which causes a viewer lessstrain associated with viewing and gives a viewer a sense of greatdepth. Another object is to provide an electronic device for enjoying animage which causes a viewer less strain associated with viewing andgives a viewer a sense of great depth.

In order to solve any of the above objects, one embodiment of thepresent invention is made with a focus on a depth effect by monocularviewing. This leads to a display device having a structure exemplifiedin this specification.

In addition, the present inventors found that a dark portion in an imageseen by a viewer is expressed more precisely, thereby giving the image agreater sense of depth. Then, the present inventors conceived to add acircuit for converting an image contained in an image signal (alsoreferred to as image source) which is inputted to a display device ofone embodiment of the present invention to an image giving a viewer agreater sense of depth.

When a viewer sees a real object, the viewer can perceive a texture ofthe surface of the object and can experience a stereoscopic effect evenby monocular viewing, by reflection of light and dark on a surface ofthe object. Further, the viewer can perceive the positional relationbetween the object and the backdrop from luminance gradient in a shadedportion which is a light-blocked portion by the object and focused ontothe backdrop. Accordingly, in an image displayed in the display device,a dark portion is particularly expressed more precisely, therebydisplaying images which can give a viewer a rich sense of depth.

A conventional image signal has a given grayscale and representsgrayscale linearly from the lowest luminance to the highest luminance inaccordance with a dynamic range of luminance a display device can have.However, in a conventional image expressed by a constant luminanceinterval without distinction between a dark portion and a light portion,luminance difference of a dark portion cannot be reproduced precisely;thus, such an image is difficult to give a viewer a sense of depth andperceived as a flat image.

Thus, an image signal having a given grayscale needs to be convertedinto an image signal capable of representing an image on the lowluminance side by high-level grayscale.

Thus, one embodiment of the present invention is a display deviceincluding a display portion and a grayscale conversion portion. Thedisplay portion has an NTSC ratio of 80% or higher and a contrast ratioof 500 or higher. In the display portion, pixels each including alight-emitting module capable of emitting light with a spectral linehalf-width of 60 nm or less within a response time of 100 μs areprovided at a resolution of 80 ppi or more. The grayscale conversionportion converts a first image signal inputted to the grayscaleconversion portion into a second image signal and transmits the secondimage signal to the display portion. The first image signal has p graylevels (p is a natural number). The second image signal is subjected tograyscale conversion processing in which each q gray levels from thelowest luminance to the q-th value in the first image signal is furtherdivided into 2^(r) value. Here, q is greater than or equal to 1 and lessthan or equal to p/2 and r is greater than or equal to 1 and less thanor equal to 20.

In the display portion included in such a display device, thedistribution of light and shade in an image can be widened and thus adetailed image can be displayed. Further, an image which is faithful tocamerawork can be displayed smoothly. Accordingly, a viewer is given agreater sense of depth by monocular vision, which can eliminate the needfor displaying images including a binocular parallax on one screen. Inaddition, a viewer can see an image with naked eyes. As a result, it ispossible to reduce strain on a viewer which is caused by viewing and todisplay an image which allows a viewer to have a great sense of depth.

Further, the pixel includes the light-emitting module which emits lightwith a narrow spectral line half-width and high color purity; therefore,the NTSC ratio is high and the contrast is high. Thus, an image with awide grayscale range can be displayed. Since the pixel includes thelight-emitting element having a short response time, an image in motioncan be displayed smoothly. Thus, a moving image in which a front imagemoves smoothly and faster than a back image while overlapping with theback image can be expressed. The wide grayscale range and the smoothmotion interact with each other, which allows a viewer to see an imagewith a strong sense of depth.

Further, the number of gray levels is increased in the low luminanceregions using the grayscale conversion portion, whereby a dark portioncan be precisely expressed. Combining with the above-described displayportion can synergistically widen the distribution of light and shade inan image; thus, a texture of a surface of an object, shading of a shadedportion formed by the object, and the like can be faithfully reproduced.Thus, an image which can give a viewer a rich sense of depth can bedisplayed.

Another embodiment of the present invention includes a display portionand a grayscale conversion portion. The display portion has an NTSCratio of 80% or higher and a contrast ratio of 500 or higher. In thedisplay portion, pixels each including a light-emitting module capableof emitting light with a spectral line half-width of 60 nm or lesswithin a response time of 100 μs are provided at a resolution of 80 ppior more. The light-emitting module includes a reflective film; asemi-transmissive and semi-reflective film; a light-emitting elementprovided between the reflective film and the semi-transmissive andsemi-reflective film, and including a pair of electrodes, a plurality oflayers containing a light-emitting organic compound between the pair ofelectrodes, and an interlayer between the layers containinglight-emitting organic compounds; and a color filter overlapping withthe light-emitting element with the semi-transmissive andsemi-reflective film provided therebetween. The grayscale conversionportion converts a first image signal inputted to the grayscaleconversion portion into a second image signal and transmits the secondimage signal to the display portion. The first image signal has p graylevels (p is a natural number). The second image signal is subjected tograyscale conversion processing in which each q gray levels from thelowest luminance to the q-th value is further divided into 2^(r) value.Here, q is greater than or equal to 1 and less than or equal to p/2 andr is greater than or equal to 1 and less than or equal to 20.

With such a structure, light interferes with each other between thereflective film and the semi-transmissive and semi-reflective film, aspecific light among light with a wavelength of 400 nm or higher andlower than 800 nm is strengthened, and unnecessary light are absorbed bythe color filter. Accordingly, high color saturation images can bedisplayed by light with a narrow spectral line width (specifically, 60nm or less), thereby giving a viewer a greater sense of depth.Consequently, it is possible to provide a display device that candisplay images which cause a viewer less strain associated with viewingand give a viewer a sense of great depth.

In any of the display devices according to another embodiment of thepresent invention, the grayscale conversion portion divides an imagecontained in a first image signal into a plurality of regions, extractsluminance distribution in each region, determines whether to performgrayscale conversion processing in each region based on the luminancedistribution, and performs grayscale conversion processing to the regionwhich needs the grayscale conversion processing.

With such a structure, grayscale conversion processing is performed notin all the regions of the image contained in the first image signal butin the region which needs the grayscale conversion processing. Thus,processing rate in the grayscale conversion portion can be increased.Further, since the grayscale conversion portion does not performgrayscale conversion processing in a region which does not need theprocessing, driving power consumption of the display device can bereduced.

In any of the display devices according to another embodiment of thepresent invention, a light-emitting module provided in a pixel is afirst light-emitting module including a color filter transmitting lightexhibiting red and a reflective film and a semi-transmissive andsemi-reflective film between which the optical path length is adjustedto i/2 times (i is a natural number) the length greater than or equal to600 nm and less than 800 nm, a second light-emitting module including acolor filter transmitting light exhibiting green and a reflective filmand a semi-transmissive and semi-reflective film between which theoptical path length is adjusted to j/2 times (j is a natural number) thelength greater than or equal to 500 nm and less than 600 nm, or a thirdlight-emitting module including a color filter transmitting lightexhibiting blue and a reflective film and a semi-transmissive andsemi-reflective film between which the optical path length is adjustedto k/2 times (k is a natural number) the length greater than or equal to400 nm and less than 500 nm.

Further, in any of the display devices according to another embodimentof the present invention, a light-emitting module provided in a pixel isa first light-emitting module including a color filter transmittinglight exhibiting red and a reflective film and a semi-transmissive andsemi-reflective film between which the optical path length is adjustedto i/2 times (i is a natural number) the length greater than or equal to600 nm and less than 800 nm, a second light-emitting module including acolor filter transmitting light exhibiting green and a reflective filmand a semi-transmissive and semi-reflective film between which theoptical path length is adjusted to j/2 times (j is a natural number) thelength greater than or equal to 500 nm and less than 600 nm, or a thirdlight-emitting module including a color filter transmitting lightexhibiting blue and a reflective film and a semi-transmissive andsemi-reflective film between which the optical path length is adjustedto k/2 times (k is a natural number) the length greater than or equal to400 nm and less than 500 nm. The first light-emitting module, the secondlight-emitting module, and the third light-emitting module share onelayer containing a light-emitting organic compound.

In any of the above-described display devices according to anotherembodiment of the present invention, the light-emitting module in whichone of the pair of electrodes also serves as a reflective film and theother also serves as a semi-transmissive and semi-reflective film.

With such a structure, color purity of light emitted from each of thelight-emitting module can be increased. Further, the layers containing alight-emitting organic compound can be formed in one step. Further, thepair of electrodes also can serve as the reflective film and thesemi-transmissive and semi-reflective film. Therefore, a manufacturingprocess can be simplified. Thus, it is possible to provide a displaydevice that can be easily manufactured and display an image which causesa viewer less strain associated with viewing and gives a viewer a senseof great depth.

In particular, a microcavity is highly effective in narrowing thespectral line half-width and in making a pixel become more unnoticeableas the resolution becomes higher. Further, it is easy for a human brainto recognize an image in motion and an image which changes from a stillimage to a moving image. Therefore, by increasing color purity andmaking a pixel become more unnoticeable, a smoother moving image can bedisplayed; thus, it is possible to provide a display device that candisplay an image which causes a viewer less strain associated withviewing and gives a viewer a sense of great depth.

In any of the display devices according to another embodiment of thepresent invention, a light-emitting module provided in a pixel emitslight exhibiting red with a spectral line half-width of less than 50 nm,light exhibiting green with a spectral line half-width of narrower thanthe spectral line half-width of light exhibiting red, or lightexhibiting blue with a spectral line half-width is narrower than thespectral line half-width of light exhibiting green.

In such a structure, the half-width of light exhibiting green, whoseluminosity is higher than that of light exhibiting red, is narrower thanthe half-width of light exhibiting red, and the half-width of lightexhibiting blue is narrower than the half-width of light exhibitinggreen. Thus, an image with high saturation can be displayed with the useof light having a narrow spectral line half-width (specifically 50 nm orless), and a depth effect is enhanced.

One embodiment of the present invention is an electronic deviceincluding any of the above display devices.

According to one embodiment of the present invention, an image with awide distribution of light and shade is displayed on the electronicdevice. Further, an image which is faithful to camerawork can bedisplayed smoothly on the electronic device. Therefore, a viewer isgiven a greater sense of depth by monocular viewing, which can eliminatethe need for displaying images including a binocular parallax on onescreen. In addition, a viewer can see an image with naked eyes. Thus, itis possible to provide an electronic device for enjoying images whichcause a viewer less strain associated with viewing and give a viewer asense of great depth.

Note that “optical path length” in this specification means the productof distance and refractive index. Therefore, the optical path length ofa medium having a refractive index of more than 1 is longer than theactual distance. Note that the optical path length in a resonator of amicro resonator (also referred to as microcavity) can be obtained bymeasuring optical interference. Specifically, the optical path length ina resonator can be obtained as follows: an intensity ratio of reflectedlight to incident light is measured with a spectrophotometer and themeasured intensity ratio is plotted with respect to a wavelength.

Note that in this specification, the display device includes any of thefollowing modules in its category: a module in which a connector such asa flexible printed circuit (FPC) or a tape carrier package (TCP) isattached to a display panel; a module having a TCP provided with aprinted wiring board at the end thereof; and a module having anintegrated circuit (IC) directly mounted over a substrate over which adisplay portion is formed by a chip on glass (COG) method.

According to the present invention, a display device that can display animage which causes a viewer less strain associated with viewing andgives a viewer a sense of great depth can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a display device according toEmbodiment.

FIGS. 2A to 2C are diagrams illustrating a display portion of a displaydevice according to Embodiment.

FIG. 3 is a diagram illustrating an image processing device according toEmbodiment.

FIGS. 4A to 4C are diagrams illustrating grayscale conversion processingaccording to Embodiment.

FIGS. 5A to 5D are diagrams illustrating grayscale conversion processingaccording to Embodiment.

FIGS. 6A to 6D are diagrams illustrating grayscale conversion processingaccording to Embodiment.

FIGS. 7A to 7C are diagrams illustrating a light-emitting element thatcan be applied to a display device according to Embodiment.

FIGS. 8A to 8E each illustrate an electronic device including a displaydevice according to Embodiment.

FIG. 9 illustrates a structure of a light-emitting element included in alight-emitting module according to Example.

FIGS. 10A to 10C are graphs each showing an emission spectrum of alight-emitting module according to Example.

FIG. 11 is a chromaticity diagram in which colors of light emitted fromlight-emitting modules according to Example are plotted.

FIGS. 12A and 12B are graphs showing time dependence of luminanceemitted from light-emitting modules according to Example.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the present invention will be described in details withreference to the drawings. Note that the invention is not limited to thefollowing description, and it will be easily understood by those skilledin the art that various changes and modifications can be made withoutdeparting from the spirit and scope of the invention. Therefore, theinvention should not be construed as being limited to the description inthe following embodiments. Note that in the structures of the inventiondescribed below, the same portions or portions having similar functionsare denoted by the same reference numerals in different drawings, anddescription of such portions is not repeated.

Note that in each drawing described in this specification, the size, thelayer thickness, or the region of each component is exaggerated forclarity in some cases. Therefore, embodiments of the present inventionare not limited to such scales.

Embodiment 1

In this embodiment, a configuration example of a display device of oneembodiment of the present invention is described with reference todrawings.

FIG. 1 is a block diagram illustrating a configuration example of adisplay device according to one embodiment of the present invention.

A display device 100 includes a display portion 101, a driver circuitportion 103, a driver circuit portion 104, a control circuit portion105, an image processing device 107, and a decoder circuit portion 109.

The decoder circuit portion 109 is inputted with an image signal S0which is compressed or encoded, decodes the image signal S0, andconverts it into a first image signal S1. The first image signal S1converted here is transmitted to the image processing device 107.

The image processing device 107 converts the inputted first image signalS1 into a second image signal S2 containing image data which is properto display in the display portion 101.

The image processing device 107 includes at least a grayscale conversionportion 111. The grayscale conversion portion 111 performs grayscaleconversion processing to the first image signal S1 having p gray levels,thereby generating the second image signal S2. In the grayscaleconversion processing, grayscale conversion processing in which each ofq gray levels from the lowest luminance to the q-th value in the firstimage signal S1 is further divided into 2^(r) value. Here, q is greaterthan or equal to 1 and less than or equal to p/2 and r is greater thanor equal to 1 and less than or equal to 20.

Note that the image processing device 107 is described in detail in anembodiment below.

The second image signal S2 converted in the image processing device 107is transmitted to the control circuit portion 105.

The control circuit portion 105 transmits a driving signal in accordancewith the second image signal S2 to each of the driver circuit portion103 and the driver circuit portion 104 which drive the display portion101.

The control circuit portion 105 may include, for example, a DAconverter, an amplifier circuit, a register circuit, and the like.

The driver circuit portions 103 and 104 drive pixels in the displayportion 101 in accordance with the driving signal inputted from thecontrol circuit portion 105 and display images in the display portion101.

The display portion 101 can display images which are eye-friendly andgive viewers a great sense of depth.

<Structure Example of Display Portion>

A structure example of the display portion 101 is described in detailbelow.

A display panel 400 exemplified in this embodiment includes the displayportion 101 over a first substrate 410. The display portion 101 includesa plurality of pixels 402. The pixel 402 includes a plurality ofsub-pixels (e.g., three sub-pixels) (FIG. 2A). Over the first substrate410, in addition to the display portion 101, a source side drivercircuit portion 104 and a gate side driver circuit portion 103 whichdrive the display portion 101 are provided. Note that the driver circuitportions can be provided not over the first substrate 410 butexternally.

The display panel 400 includes an external input terminal and receives avideo signal, a clock signal, a start signal, a reset signal, and thelike from an FPC (flexible printed circuit) 409. Note that although onlyan FPC is illustrated here, a printed wiring board (PWB) may be attachedthereto. The display panel in this specification includes not only amain body of the display panel but one with an FPC or a PWB attachedthereto.

A sealant 405 bonds the first substrate 410 and a second substrate 440.The display portion 101 is sealed in a space 431 formed between thesubstrates (see FIG. 2B).

<Structure of Display Panel>

The structure including the cross-sectional view of the display panel400 is described with reference to FIG. 2B. The display panel 400includes the source side driver circuit portion 104, a sub-pixel 402Gincluded in the pixel 402, and a lead wiring 408. Note that the displayportion 101 of the display panel 400 exemplified in this embodimentemits light in the direction denoted by the arrow in the drawing,thereby displaying images.

A CMOS circuit, which is a combination of an n-channel transistor 413and a p-channel transistor 414, is formed for the source side drivercircuit portion 104. Note that the driver circuit is not limited to thisstructure and may be various circuits, such as a CMOS circuit, a PMOScircuit, or an NMOS circuit.

The lead wiring 408 transmits a signal inputted from an external inputterminal to the source side driver circuit portion 104 and the gate sidedriver circuit portion 103.

The sub-pixel 402G includes a switching transistor 411, a currentcontrol transistor 412, and a light-emitting module 450G. Note that aninsulating layer 416 and a partition 418 are formed over the transistor411 and the like. The light-emitting module 450G includes a reflectivefilm, a semi-transmissive and semi-reflective film, a light-emittingelement 420G between the reflective film and the semi-transmissive andsemi-reflective film, and a color filter 441G provided on thesemi-reflective film side through which light emitted from thelight-emitting element 420G is extracted. In the light-emitting module450G exemplified in this embodiment, a first electrode 421G and a secondelectrode 422 of the light-emitting element 420G also serve as thereflective film and the semi-transmissive and semi-reflective film,respectively. Note that a direction of an image displayed in the displayportion 101 is determined in accordance with a direction in which lightemitted from the light-emitting element 420G is extracted.

In addition, a light-blocking film 442 is formed so as to surround thecolor filter 441G. The light-blocking film 442 prevents a phenomenon inwhich the display panel 400 reflects outside light and has an effect ofincreasing the contrast of images displayed in the display portion 101.Note that the color filter 441G and the light-blocking film 442 areformed on the second substrate 440.

The insulating layer 416 is a layer having insulating properties forplanarizing a step due to the structure of the transistor 411 and thelike or for suppressing impurity dispersion to the transistor 411 andthe like. The insulating layer 416 can be a single layer or a stackedlayer. The partition 418 is an insulating layer having an opening; thelight-emitting element 420G is formed in the opening of the partition418.

The light-emitting element 420G includes the first electrode 421G, thesecond electrode 422, and a layer 423 containing a light-emittingorganic compound.

Structure of Transistor

Top-gate transistors are used in the display panel 400 exemplified inFIG. 2A. Various types of transistors can be used for the source sidedriver circuit portion 104, the gate side driver circuit portion 103,and the sup-pixels. Note that various semiconductors can be used for aregion where channels of these transistors are formed. Specifically,amorphous silicon, polysilicon, single crystal silicon, an oxidesemiconductor, or the like can be used.

When an oxide semiconductor is used for a region where a channel of atransistor is formed, the transistor can be smaller than a transistor inwhich an amorphous silicon is used, which results in higher resolutionpixels in a display portion.

An oxide semiconductor film which can be used for a semiconductor layerof a transistor may be in a non-single-crystal state, for example. Thenon-single-crystal state is, for example, structured by at least one ofc-axis aligned crystal (CAAC), polycrystal, microcrystal, and anamorphous part. The density of defect states of an amorphous part ishigher than those of microcrystal and CAAC. The density of defect statesof microcrystal is higher than that of CAAC. Note that an oxidesemiconductor including CAAC is referred to as a CAAC-OS (c-axis alignedcrystalline oxide semiconductor).

For example, an oxide semiconductor film may include a CAAC-OS. In theCAAC-OS, for example, c-axes are aligned, and a-axes and/or b-axes arenot macroscopically aligned.

For example, an oxide semiconductor film may include microcrystal. Notethat an oxide semiconductor including microcrystal is referred to as amicrocrystalline oxide semiconductor. A microcrystalline oxidesemiconductor film includes microcrystal (also referred to asnanocrystal) with a size greater than or equal to 1 nm and less than 10nm, for example.

For example, an oxide semiconductor film may include an amorphous part.Note that an oxide semiconductor including an amorphous part is referredto as an amorphous oxide semiconductor. An amorphous oxide semiconductorfilm, for example, has disordered atomic arrangement and no crystallinecomponent. Alternatively, an amorphous oxide semiconductor film is, forexample, absolutely amorphous and has no crystal part.

Note that an oxide semiconductor film may be a mixed film including anyof a CAAC-OS, a microcrystalline oxide semiconductor, and an amorphousoxide semiconductor. The mixed film, for example, includes a region ofan amorphous oxide semiconductor, a region of a microcrystalline oxidesemiconductor, and a region of a CAAC-OS. Further, the mixed film mayhave a stacked structure including a region of an amorphous oxidesemiconductor, a region of a microcrystalline oxide semiconductor, and aregion of a CAAC-OS, for example.

Note that an oxide semiconductor film may be in a single-crystal state,for example.

An oxide semiconductor film preferably includes a plurality of crystalparts. In each of the crystal parts, a c-axis is preferably aligned in adirection parallel to a normal vector of a surface where the oxidesemiconductor film is formed or a normal vector of a surface of theoxide semiconductor film. Note that, among crystal parts, the directionsof the a-axis and the b-axis of one crystal part may be different fromthose of another crystal part. An example of such an oxide semiconductorfilm is a CAAC-OS film.

Note that in most cases, a crystal part in the CAAC-OS film fits insidea cube whose one side is less than 100 nm. In an image obtained with atransmission electron microscope (TEM), a boundary between crystal partsin the CAAC-OS film are not clearly detected. Further, with the TEM, agrain boundary in the CAAC-OS film is not clearly found. Thus, in theCAAC-OS film, a reduction in electron mobility due to the grain boundaryis suppressed.

In each of the crystal parts included in the CAAC-OS film, for example,a c-axis is aligned in a direction parallel to a normal vector of asurface where the CAAC-OS film is formed or a normal vector of a surfaceof the CAAC-OS film. Further, in each of the crystal parts, metal atomsare arranged in a triangular or hexagonal configuration when seen fromthe direction perpendicular to the a-b plane, and metal atoms arearranged in a layered manner or metal atoms and oxygen atoms arearranged in a layered manner when seen from the direction perpendicularto the c-axis. Note that, among crystal parts, the directions of thea-axis and the b-axis of one crystal part may be different from those ofanother crystal part. In this specification, a term “perpendicular”includes a range from 80° to 100°, preferably from 85° to 95°. Inaddition, a term “parallel” includes a range from −10° to 10°,preferably from −5° to 5°.

In the CAAC-OS film, distribution of crystal parts is not necessarilyuniform. For example, in the formation process of the CAAC-OS film, inthe case where crystal growth occurs from a surface side of the oxidesemiconductor film, the proportion of crystal parts in the vicinity ofthe surface of the oxide semiconductor film is higher than that in thevicinity of the surface where the oxide semiconductor film is formed insome cases. Further, when an impurity is added to the CAAC-OS film,crystallinity of the crystal part in a region to which the impurity isadded is lowered in some cases.

Since the c-axes of the crystal parts included in the CAAC-OS film arealigned in the direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, the directions of the c-axes may be different from eachother depending on the shape of the CAAC-OS film (the cross-sectionalshape of the surface where the CAAC-OS film is formed or thecross-sectional shape of the surface of the CAAC-OS film). Note that thefilm deposition is accompanied with the formation of the crystal partsor followed by the formation of the crystal parts throughcrystallization treatment such as heat treatment. Hence, the c-axes ofthe crystal parts are aligned in the direction parallel to a normalvector of the surface where the CAAC-OS film is formed or a normalvector of the surface of the CAAC-OS film.

In a transistor using the CAAC-OS film, change in electriccharacteristics due to irradiation with visible light or ultravioletlight is small. Thus, the transistor has high reliability.

In the case where a CAAC-OS film is deposited by a sputtering method, asubstrate temperature in the deposition is preferably high. For example,an oxide film is deposited at a substrate heating temperature from 100°C. to 600° C., preferably from 200° C. to 500° C., further preferablyfrom 150° C. to 450° C., whereby a CAAC-OS film can be deposited.

Electric power used in a sputtering method is preferably supplied from adirect-current (DC) source. Note that a radio frequency (RF) powersource or an alternating-current (AC) power source can be used. Notethat it is difficult to use an RF power source for a sputteringapparatus which is capable of deposition to a large-sized substrate. Inaddition, a DC power source is preferred to an AC power source from theviewpoint below.

In the case where an In—Ga—Zn—O compound target is used as a sputteringtarget, an In—Ga—Zn—O compound target in which InO_(x) powder, GaO_(y)powder, and ZnO_(z) powder are mixed in the molar ratio of 2:2:1, 8:4:3,3:1:1, 1:1:1, 4:2:3, 3:1:2, or 3:1:4 is preferably used, for example. x,y, and z are any positive numbers. Note that a sputtering target may bepolycrystalline.

Alternatively, with use of magnetron, plasma area near a sputteringtarget can be increased in density due to a magnetic field. For example,in a magnetron sputtering apparatus, a magnetic assembly is located inthe back of the sputtering target and a magnetic field is generated inthe front of the sputtering target. When sputtering to the sputteringtarget, the magnetic field traps ionized electrons and secondaryelectrons generated by the sputtering. The electrons trapped in this wayenhance the odds of collision with an inert gas, such as a rare gas, inthe deposition chamber, thereby increasing the plasma density. Thus, thedeposition rate can be increased without significantly increasing thetemperature of an element formation layer.

In the case where a CAAC-OS film is deposited by a sputtering method,for example, impurities (e.g., hydrogen, water, carbon dioxide, andnitrogen) existing in a deposition chamber of a sputtering apparatus ispreferably reduced. Further, the concentration of impurities in adeposition gas is preferably reduced. For example, as a deposition gassuch as an oxygen gas or an argon gas, a highly purified gas having adew point of −40° C. or lower, preferably −80° C. or lower, stillpreferably −100° C. or lower is used, whereby suppressing entry ofimpurities into a CAAC-OS film.

In the case where a CAAC-OS film is deposited by a sputtering method, itis preferable to suppress plasma damage when the deposition is performedby increasing the oxygen percentage in the deposition gas and optimizingelectric power. For example, the oxygen percentage in the deposition gasis preferably 30 vol % or higher, still preferably 100 vol %.

In the case where a CAAC-OS film is deposited by a sputtering method,heat treatment may be performed in addition to the substrate heatingwhen the deposition is performed. By the heat treatment, the impurityconcentration in the oxide semiconductor film can be reduced, forexample.

The heat treatment is performed at higher than or equal to 350° C. andlower than a strain point of the substrate, or may be performed athigher than or equal to 350° C. and lower than or equal to 450° C. Notethat the heat treatment may be performed more than once.

There is no particular limitation on a heat treatment apparatus to beused for the heat treatment, and a rapid thermal annealing (RTA)apparatus such as a gas rapid thermal annealing (GRTA) apparatus or alamp rapid thermal annealing (LRTA) apparatus may be used.Alternatively, another heat treatment apparatus such as an electricfurnace may be used.

As described in the above process, an impurity concentration in theoxide semiconductor film is reduced by preventing entry of hydrogen,water, or the like into the film during the deposition. The impurityconcentration can be reduced by removing hydrogen, water, or the likecontained in the oxide semiconductor film through the heat treatmentafter the deposition of the oxide semiconductor film. After that, oxygenis supplied to the oxide semiconductor film to repair oxygen defects,thereby highly purifying the oxide semiconductor film. Moreover, oxygenmay be added to the oxide semiconductor film. The purified oxidesemiconductor is an i-type (intrinsic) semiconductor or a substantiallyi-type semiconductor. The carrier density of the oxide semiconductorfilm which is substantially i-type is lower than 1×10¹⁷/cm³, lower than1×10¹⁵/cm³, or lower than 1×10¹³/cm³.

When a single crystal semiconductor is used for a region where a channelof a transistor is formed, the size of the transistor can be reduced,which results in higher resolution pixels in a display portion.

As a single crystal semiconductor used for forming a semiconductorlayer, a semiconductor substrate, typical examples of which include asingle crystal semiconductor substrate formed using elements belongingto Group 14, such as a single crystal silicon substrate, a singlecrystal germanium substrate, or a single crystal silicon germaniumsubstrate, and a compound semiconductor substrate (e.g., a SiCsubstrate, a sapphire substrate, and a GaN substrate), can be used.Preferred one is a silicon on insulator (SOI) substrate in which asingle crystal semiconductor layer is provided on an insulating surface.

An SOI substrate can be fabricated by the following methods: afteroxygen ions are implanted in a mirror-polished wafer, the wafer isheated at high temperatures to form an oxidized layer at a predetermineddepth from a surface of the wafer and eliminate defects generated in asurface layer; the semiconductor substrate is separated by utilizing thegrowth of microvoids formed by hydrogen ion irradiation (this growth iscaused by heat treatment); or a single crystal semiconductor layer isformed on an insulating surface by crystal growth.

In this embodiment, ions are added through one surface of a singlecrystal semiconductor substrate, an embrittlement layer is formed at acertain depth from the one surface of the single crystal semiconductorsubstrate, and an insulating layer is formed over the one surface of thesingle crystal semiconductor substrate or over the first substrate 410.Next, heat treatment is performed in the state in which the singlecrystal semiconductor substrate provided with the embrittlement layerand the first substrate 410 are bonded to each other with the insulatinglayer interposed therebetween, so that a crack is generated in theembrittlement layer to separate the single crystal semiconductorsubstrate along the embrittlement layer. Thus, a single crystalsemiconductor layer, which is separated from the single crystalsemiconductor substrate, is formed as a semiconductor layer over thefirst substrate 410. Note that a glass substrate can be used as thefirst substrate 410.

Further, regions electrically insulated from each other may be formed inthe semiconductor substrate so that transistors 411 and 412 may beformed using the regions electrically insulated from each other.

When a channel formation region is formed using a single crystalsemiconductor, variations in electrical characteristics, such asthreshold voltage, between transistors due to bonding defects at grainboundaries can be reduced. Thus, in the display panel according to oneembodiment of the present invention, the light-emitting elements can beoperated normally without placing a circuit for compensating thresholdvoltage in each pixel. The number of circuit elements per pixel cantherefore be reduced, increasing the flexibility in layout. Thus, ahigh-definition display panel can be achieved. For example, a structurein which a matrix of a plurality of pixels is included, specifically 300or more pixels per inch (i.e., the horizontal resolution is 300 or morepixels per inch (ppi)), preferably 400 or more pixels per inch (i.e.,the horizontal resolution is 400 ppi or more), can be achieved.

Moreover, a transistor whose channel formation region is composed of asingle crystal semiconductor can be downsized while keeping high currentdrive capability. The use of the downsized transistor leads to areduction in the area of a circuit portion that does not contribute todisplay, which results in an increase in the display area in the displayportion and a reduction in the frame size of the display panel.

Structure of Pixel

The structure of the pixel 402 included in the display portion 101 isdescribed with reference to FIG. 2C.

The pixel 402 exemplified in this embodiment includes the sub-pixel402G. The sub-pixel 402G includes the light-emitting element 420G; thelight-emitting element 420G includes the first electrode 421G alsoserving as a reflective film, the second electrode 422 also serving as asemi-transmissive and semi-reflective film, a layer 423 a containing alight-emitting organic compound, a layer 423 b containing alight-emitting organic compound, and an interlayer 424. Further, thepixel 402 includes the color filter 441G on the second electrode 422side so as to overlap with the light-emitting element 420G, and thelight-emitting module 450G capable of emitting light with a spectrumwhose half-width is 60 nm or less and wavelength of 400 nm or higher andlower than 800 nm within a response time of 100 μs.

Such a pixel is provided in the display portion 101 at a resolution of80 ppi or higher, preferably 300 ppi or higher; thus, a display devicewith an NTSC ratio of 80% or higher, preferably 95% or higher, and acontrast ratio of 500 or higher, preferably 2000 or higher is provided.Consequently, it is possible to provide a display device that candisplay images which cause a viewer less strain associated with viewingand give a viewer a sense of great depth. In addition, light interfereswith each other between the reflective film and the semi-transmissiveand semi-reflective film, a specific light among light with a wavelengthof 400 nm or higher and lower than 800 nm is strengthened, andunnecessary light are absorbed by the color filter. Accordingly, highcolor saturation images can be displayed by light with a narrow spectralline width (specifically, 60 nm or less), thereby giving a viewer agreater sense of depth.

In addition, the pixel 402 includes a sub-pixel 402B emitting light Bexhibiting blue, a sub-pixel 402G emitting light G exhibiting green, anda sub-pixel 402R emitting light R exhibiting red. Each sub-pixelincludes a driver transistor and a light-emitting module. Eachlight-emitting module includes a reflective film, a semi-transmissiveand semi-reflective film, and a light-emitting element between thereflective film and the semi-transmissive and semi-reflective film.

When a microresonator is formed between the reflective film and thesemi-transmissive and semi-reflective film and a light-emitting elementis formed therebetween, light with a specific wavelength can beefficiently extracted through the semi-transmissive and semi-reflectivefilm. Specifically, the optical path length of the microresonator is n/2times (n is a natural number) the wavelength of extracted light; thus,light extraction efficiency can be enhanced. The wavelength of extractedlight depends on the distance between the reflective film and thesemi-transmissive and semi-reflective film, and the distance can beadjusted by forming an optical adjustment layer between the films.

A conductive film having light-transmitting properties to visible lightor a layer containing a light-emitting organic compound can be employedfor a material that can be used for the optical adjustment layer. Forexample, the thickness of the optical adjustment layer may be adjustedusing a charge generation region. Alternatively, a region containing asubstance having a high hole-transport property and an acceptorsubstance is preferably used for the optical adjustment layer because anincrease in driving voltage can be suppressed even when the opticaladjustment layer is thick.

As the structure of the light-emitting element, the light-emittingelement 420G is provided between the first electrode 421G also servingas a reflective film and the second electrode 422 also serving as asemi-transmissive and semi-reflective film. The light-emitting element420G includes the layer 423 a containing a light-emitting organiccompound, the layer 423 b containing a light-emitting organic compound,and the interlayer 424.

Note that the structure example of the light-emitting element isdescribed in detail in Embodiment 3.

Here, in the case of a display device using a liquid-crystal element ina pixel, the response time can not be increased enough because pixelsare driven by physically changing the orientation of liquid crystals. Incontrast, the response time of the above-described light-emittingelement is shorter than a liquid-crystal element. Thus, a display deviceusing such a light-emitting element can display smooth moving images, inwhich after-images do not likely appear when displaying moving images.As a result, a display device capable of displaying more vivid andstereoscopic images and giving viewers a rich sense of depth can beobtained.

The light-emitting modules exemplified in this embodiment each have astructure in which the second electrode 422 provided in thelight-emitting module also serves as a semi-transmissive andsemi-reflective film. Specifically, the second electrode 422 shared bythe light-emitting elements 420B, 420G, and 420R also serves as asemi-transmissive and semi-reflective film of the light-emitting modules450B, 450G, and 450R.

In addition, the first electrodes of the light-emitting elements each ofwhich is provided in the light-emitting modules and are electricallyseparated from each other also serve as reflective films. Specifically,a first electrode 421B provided in the light-emitting element 420B alsoserves as a reflective film of the light-emitting module 450B, the firstelectrode 421G provided in the light-emitting element 420G also servesas a reflective film of the light-emitting module 450G, and a firstelectrode 421R provided in the light-emitting element 420R also servesas a reflective film of the light-emitting module 450R.

The first electrode also serving as a reflective film of alight-emitting module has a stacked-layer structure in which an opticaladjustment layer is stacked over the reflective film. The opticaladjustment layer is preferably formed of a conductive film havinglight-transmitting properties with respect to visible light, and thereflective film is preferably formed of a conductive metal film havinghigh reflectivity with respect to visible light.

The thickness of the optical adjustment layer is adjusted in accordancewith a wavelength of light extracted from a light-emitting module.

For example, the first light-emitting module 450B includes a colorfilter 441B which transmits light exhibiting blue, the first electrode421B also serving as a reflective film, and the second electrode 422also serving as a semi-transmissive and semi-reflective film; theoptical path length between the first electrode 421B and the secondelectrode 422 is adjusted to k/2 times (k is a natural number) thelength greater than or equal to 400 nm and less than 500 nm.

Further, the second light-emitting module 450G includes a color filter441G which transmits light exhibiting green, a reflective film, and asemi-transmissive and semi-reflective film; the optical path lengthbetween the reflective film and the semi-transmissive andsemi-reflective film is adjusted to j/2 times (j is a natural number)the length greater than or equal to 500 nm and less than 600 nm.

Further, the third light-emitting module 450R includes a color filter441R which transmits light exhibiting red, a reflective film, and asemi-transmissive and semi-reflective film; the optical path lengthbetween the reflective film and the semi-transmissive andsemi-reflective film is adjusted to i/2 times (i is a natural number)the length greater than or equal to 600 nm and less than 800 nm.

In such a light-emitting module, light emitted from the light-emittingelements interfere with each other between the reflective film and thesemi-transmissive and semi-reflective film, light having a specificwavelength among light having a wavelength of greater than or equal to400 nm and less than 800 nm is strengthened, and the color filterabsorbs unnecessary light. Accordingly, high color saturation images canbe displayed by light with a narrow spectral line width (specifically,60 nm or less), thereby giving a viewer a greater sense of depth.Consequently, it is possible to provide a display device that candisplay images which cause a viewer less strain associated with viewingand give a viewer a sense of great depth.

In particular, the third light-emitting module 450R emits lightexhibiting red with a spectral line half-width of less than 50 nm, thesecond light-emitting module 450G emits light exhibiting green with aspectral line half-width of smaller than that of the light emitted fromthe third light-emitting module 450R, and the first light-emittingmodule 450B emits light exhibiting blue with a spectral line half-widthof smaller than that of the light emitted from the second light-emittingmodule 450G.

In the light-emitting module with such a structure, a half-width ofhigh-luminosity green light is narrower than that of red light and ahalf-width of blue light is narrower than that of green light.Accordingly, high color saturation images can be displayed by light witha narrow spectral line width (specifically, 50 nm or less), therebygiving a viewer a greater sense of depth.

Note that the first light-emitting module 450B, the secondlight-emitting module 4506, and the third light-emitting module 450Reach include the layer 423 a containing a light-emitting organiccompound, the layer 423 b containing a light-emitting organic compound,and the interlayer 424. In addition, one of the pair of electrodes ofthe light-emitting element also serves as a reflective film and theother thereof also serves as a semi-transmissive and semi-reflectivefilm.

In the light-emitting modules with such a structure, each layercontaining light-emitting organic compound in the plurality oflight-emitting modules can be formed in one process. In addition, a pairof electrodes also serves as a reflective film and a semi-transmissiveand semi-reflective film. Therefore, a manufacturing process can besimplified. Thus, it is possible to provide a display device that can beeasily manufactured and display an image which causes a viewer lessstrain associated with viewing and gives a viewer a sense of greatdepth.

In addition, the pixel includes light-emitting modules each emittinglight with a narrow spectral line half-width and high color purity,which increases NTSC ratio and contrast. Thus, an image with a widegrayscale range can be displayed. Further, a response time of thelight-emitting element in the pixel is short; thus, smooth moving imagescan be displayed. Thus, a moving image in which a front image movessmoothly and faster than a back image while overlapping with the backimage can be expressed. The wide grayscale range and the smooth motioninteract with each other, which allows a viewer to see an image with astrong sense of depth.

In particular, a microcavity is highly effective in narrowing thespectral line half-width and in making a pixel become more unnoticeableas the resolution becomes higher. Further, it is easy for a human brainto recognize an image in motion and an image which changes from a stillimage to a moving image. Therefore, by increasing color purity andmaking a pixel become more unnoticeable, a smoother moving image can bedisplayed; thus, it is possible to provide a display device that candisplay an image which causes a viewer less strain associated withviewing and gives a viewer a sense of great depth.

Structure of Partition

The partition 418 is formed to cover end portions of the firstelectrodes 421B, 421G, and 421R.

The partition 418 has a curved surface with curvature at a lower endportion thereof. As a material of the partition 418, negative orpositive photosensitive resin can be used.

Note that using a material absorbing visible light for the partitionproduces an effect of suppressing light leakage into adjacentlight-emitting elements (also called cross talk).

In addition, in such a structure that images are displayed by extractinglight emitted from the light-emitting module from the first substrate410 side which is provided with a semi-transmissive and semi-reflectivefilm, the partition formed using a material absorbing visible lightabsorbs outside light which is reflected by the reflective film on thefirst substrate 410, thereby suppressing the reflection.

Sealing Structure

The display panel 400 exemplified in this embodiment has a structure inwhich the light-emitting element is sealed in a space enclosed by thefirst substrate 410, the second substrate 440, and the sealant 405.

The space can be filled with an inert gas (e.g., nitrogen or argon) orresin. An absorbent of impurity (typically, water and/or oxygen) such asa dry agent may be provided.

The sealant 405 and the second substrate 440 are desirably formed usinga material which does not transmit impurities in the air (such as waterand/or oxygen) as much as possible. An epoxy-based resin, glass frit, orthe like can be used for the sealant 405.

Examples of the second substrate 440 include a glass substrate; a quartzsubstrate; a plastic substrate formed of polyvinyl fluoride (PVF),polyester, an acrylic resin, or the like; a substrate offiberglass-reinforced plastics (FRP); and the like.

The above is the description of the display panel 400 including thedisplay portion 101.

<Structural Example of Image Processing Device>

The following describes details on the image processing device 107exemplified in FIG. 1, with reference to drawings.

FIG. 3 is a block diagram of the image processing device 107. The imageprocessing device 107 included in the display device 100 has at leastthe grayscale conversion portion 111.

As shown in FIG. 3, the image processing device 107 may include a memoryportion 113, a noise removing portion 114, a pixel-to-pixelcomplementary portion 115, a tone correction portion 116, acomplementary frame generating portion 117, and the like.

The grayscale conversion portion 111 performs the above-describedgrayscale conversion processing on the first image signal S1. A specificexample of the grayscale conversion processing is described later.

The memory portion 113 is configured to temporarily store an initialimage data contained in the inputted first image signal S1 and imagedata subjected to conversion processing in each portion. As the memoryportion 113, a memory device such as a dynamic random access memory(DRAM), a static random access memory (SRAM), or a register circuit canbe used.

The noise removing portion 114 removes various noise, such as mosquitonoise which appears near outline of texts and the like, block noisewhich appears in high-speed moving images, random noise causing flicker,and dot noise caused by up-conversion of resolution.

The pixel-to-pixel complementary portion 115 complements data which doesnot actually exists when resolution is up-converted. For example,referring pixels around the target pixel, data is complemented todisplay intermediate color between the pixels.

The tone correction portion 116 can correct color tone of images. Forexample, the tone correction portion 116 detects a type, luminance,color purity, and the like of a lighting in a space where the displaydevice is provided, and corrects tone of images displayed in the displayportion 101 to optimal tone in accordance with the detection. The tonecorrection portion 116 can have a function of referring a displayedimage to various images of various scenes in an image list stored inadvance, and then correcting tone of the displayed image to tonesuitable to the images in the closest scene of the image.

The complementary frame generating portion 117 generates an image of aninsufficient frame due to increased frame frequency of displayed images.For example, the complementary frame generating portion 117 generates animage of a frame between two images from the difference between the twoimages, or can generate a plurality of images between two images. Forexample, when frame frequency of the first image signal S1 is 60 Hz, aplurality of complementary frames are generated and frame frequency ofthe second image signal S2 can be increased twofold (120 Hz), fourfold(240 Hz), eightfold (480 Hz), or the like.

In this manner, by adding various functions to the image processingdevice 107 other than grayscale conversion processing by the grayscaleconversion portion 111, the image processing device 107 can generatesthe second image signal S2 containing a more vivid image.

<Grayscale Conversion Processing>

The following is a description on grayscale conversion processing by thegrayscale conversion portion 111. Here, the description is made assumingthat the first image signal S1 is inputted to the grayscale conversionportion 111 and the second image signal S2 is outputted as a signalsubjected to grayscale conversion processing by the grayscale conversionportion 111.

FIG. 4A schematically shows grayscale data contained in the first imagesignal S1 inputted to the grayscale conversion portion 111. FIG. 4Bschematically shows grayscale data contained in the second image signalS2 subjected to grayscale conversion processing. FIG. 4C shows grayscaledata in which the gray level in FIG. 4B is enlarged.

The first image signal S1 has p gray levels. For example, in the casewhere the first image signal S1 is an 8-bit signal, the first imagesignal S1 has 256 gray levels. Here, when a gray level is 1, display isperformed with the lowest luminance in the dynamic range of luminancewhich can be displayed by the display device 100. On the other hand,when a gray level is p, display is performed with the highest luminance.

The gray levels of the first image signal S1 are divided at equal spacesfrom 1 to p. Thus, when the first image signal S1 is directly inputtedto the display device 100, an image is displayed by a linear grayscaleexpression from the lowest luminance to the highest luminance.

As shown in FIG. 4B, grayscale conversion processing further divides onegray level in the low luminance side. Specifically, each of q graylevels from 1 to q is divided into some levels. Here, q is from 1 to ¼p,preferably from 1 to ½p. The grayscale conversion processing isperformed in a wider range; thus, images can give a viewer a greatersense of depth.

In addition, when one gray level is divided into some levels, thedivision number is preferably factorial of 2 as shown in FIG. 4C. Thatis, one gray level is divided into 2^(r)-value. Here, r is a naturalnumber from 1 to 8, preferably from 1 to 20. As the division number ofone gray level is increased, an image given a greater sense of depth canbe generated.

For example, when the first image signal S1 represents 8-bit graylevels, q is ¼, and r is 8, each 64 values in the low luminance side ofthe first image signal S1 is divided into 256 values. In this case, thenumber of the gray levels of the second image signal S2 becomes 16512gray levels.

Here, when an organic EL element is preferably used as a light-emittingelement included in the display portion 101, more precise grayscaleexpression can be obtained in the low luminance region. Furthermore,much more precise grayscale expression can be obtained when alight-emitting module in which a microresonator (also referred to as amicrocavity) structure and a color filter are combined is used as alight-emitting module to obtain light emission with increased colorpurity from each pixel. For example, grayscale can be expressed by1000000 gray levels or more in a luminance region lower than half of themaximum luminance.

Next, new gray levels obtained by dividing one gray level are assignedto each pixel. For example, referring to the gray levels assigned topixels around a target pixel by the first image signal, an intermediategray level between the gray levels is given. Alternatively, referring tothe gray levels of a target pixel in a plurality of images of around atarget frame or to the gray level of pixels around the target pixel, anintermediate gray level between the gray levels can be given.

By the above grayscale conversion processing, the grayscale conversionportion 111 can generate the second image signal S2 containing an imagegiven a greater sense of depth from the inputted first image signal S1.

Here, images before and after grayscale conversion processing isperformed are exemplified in FIGS. 5A to 5D.

FIG. 5A is an example of an image before grayscale conversion processingis performed; the night view of skyscrapers and fireworks sent up in theback side of the skyscrapers. FIG. 5B is a histogram (also referred toas luminance distribution) of the number of pixels with respect to thegray levels of the image of FIG. 5A. Note that in the histogram of FIG.5B, the low luminance side and the high luminance side are partly shown.

In FIG. 5A, the gray level of skyscrapers located in front of thefireworks is close to the gray level of the background; thus, there aremany regions displayed by similar gray level as shown in FIG. 5B. As aresult, the positional relation (front-and-back position) between thebackground and the skyscrapers become blurry and the image is thus veryflat.

FIG. 5C is an example of the image of FIG. 5A after grayscale conversionprocessing is performed. FIG. 5D is a histogram of the number of pixelswith respect to the gray levels of the image of FIG. 5C.

As shown in FIG. 5D, the number of gray levels in the low luminanceregion is increased by grayscale conversion processing; thus, the regionexpressed by similar gray levels in FIG. 5B can be expressed moreprecisely. As a result, in the image of FIG. 5C, the positionalrelations (front-and-back position) between the background and theskyscrapers and between the skyscrapers are clearly expressed and theimage gives viewers a greater sense of depth on the whole.

The above is the description of grayscale conversion processing.

The display device 100 of one embodiment of the present invention candisplay images in the display portion 101 which are capable of givingviewers a greater sense of depth. Further, the image processing device107 including the grayscale conversion portion 111 configured to performthe grayscale conversion processing is provided in the display device100; thus, the display device 100 can display images which are given agreater sense of depth.

This embodiment can be combined with any of the other embodimentsdisclosed in this specification as appropriate.

Embodiment 2

In this embodiment, a method in which an image contained in an inputtedimage signal is divided into a plurality of regions and grayscaleconversion processing is performed only to the region which needs theprocessing is described.

An image contained in the first image signal S1 inputted to thegrayscale conversion portion 111 is divided into a plurality of regions,and each luminance distribution in the regions is extracted. Inaddition, the grayscale conversion portion 111 determines whether toperform grayscale conversion processing to the regions based on theextracted luminance distribution.

The grayscale conversion portion 111 performs the above-describedgrayscale conversion processing to a region which is determined to besubjected to grayscale conversion processing, whereas grayscale in aregion where grayscale conversion processing is not performed remainsunchanged.

Then, the grayscale conversion portion 111 integrates the regions wheregrayscale conversion processing is performed and not performed into oneimage, thereby generating the second image signal S2, and outputs thesecond image signal S2.

In this manner, grayscale conversion processing is performed not to allthe regions of the image contained in the first image signal S1 but tothe region which needs the processing; thus, the processing rate can beincreased.

A more specific example is described below with reference to drawings.

FIG. 6A shows an example of an image contained in the first image signalS1 inputted to the grayscale conversion portion 111. In the image shownin FIG. 6A, a high gray level (high luminance) region and a low graylevel (low luminance) region exist together.

Here, an example in which one image is divided into 12 regions (3 high×4wide) is described as in FIG. 6A. Addresses in a vertical direction aredenoted by A, B, and C from the top of the drawing. Addresses in ahorizontal direction are denoted by 1, 2, 3, and 4 from the left of thedrawing. One region is denoted by, for example, a region [A-1], usingthese addresses.

FIG. 6C is a histogram of a region [C-3] in FIG. 6A. The region [C-3]contains many display regions where gray levels are low.

FIG. 6D is a histogram of a region [A-2] in FIG. 6A. The region [A-2]contains few display regions where gray level is low.

With use of such a histogram extracted from each region in this manner,whether to perform grayscale conversion processing to a target region isdetermined.

One example of a method for determining is described. In the case wherea gray level which exceeds the predetermined number of pixels s existsin the range of gray levels from 1 to q in one target region, grayscaleconversion processing is performed in this region. On the other hand,when such a gray level does not exist in a target region, grayscaleconversion processing is not performed in the region.

The number of pixels s which serves as an index of determining whetherto perform grayscale conversion processing is set as appropriate inaccordance with the number of gray levels of the first image signal S1or the number of pixels included in one region.

In the histogram shown in FIG. 6C, gray levels exceeding thepredetermined number of pixels s exist in the range of gray levels from1 to q. Thus, grayscale conversion processing is performed to the region[C-3].

On the other hand, in the histogram shown in FIG. 6D, gray levelsexceeding the predetermined number of pixels s do not exist in the rangeof gray levels from 1 to q. Thus, grayscale conversion processing is notperformed to the region [A-2].

The results of determining whether to perform grayscale conversionprocessing to each region by the above method are shown in FIG. 6B.Among the regions in FIG. 6B, grayscale conversion processing isperformed only in hatched regions (region [B-2], region [B-3], region[B-4], region [C-1], region [C-2], and region [C-3]).

Here, there is no limitation on a method for determining whether toperform grayscale conversion processing. For example, whether to performgrayscale conversion processing is determined in each region based onwhether or not the total number of pixels contained in the range of graylevels from 1 to q exceeds the predetermined number of pixels.

In such a method, the predetermined number of pixels which serves as anindex of determination can be set based on the number of pixels includedin one region, without depending on the number of gray levels containedin the first image signal S1; thus, the first image signal S1 containingdifferent number of gray levels can be easily used.

Although one image is divided into 12 regions in the above-describedexample, there is no limitation on the division number. When thedivision number is increased and one image is divided into many regions,a regions which needs grayscale conversion processing can be determinedin more detail, thereby further increasing processing rate.

By the method exemplified in this embodiment, grayscale conversionprocessing can be performed only in a region which needs the processing,whereby the processing rate of the grayscale conversion portion 111 canbe increased.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

Embodiment 3

In this embodiment, the structure of a light-emitting element which canbe used for the light-emitting module according to an embodiment of thepresent invention is described with reference to FIGS. 7A to 7C.

The light-emitting element described in this embodiment as an exampleincludes a first electrode, a second electrode, and a layer containing alight-emitting organic compound (hereinafter referred to as an EL layer)provided between the first electrode and the second electrode. One ofthe first electrode and the second electrode functions as an anode, andthe other functions as a cathode. The EL layer is provided between thefirst electrode and the second electrode, and a structure of the ELlayer may be appropriately selected in accordance with materials of thefirst electrode and second electrode.

<Structural Example of Light-Emitting Element>

An example of the structure of the light-emitting element is illustratedin FIG. 7A. The light-emitting element exemplified in FIG. 7A includesan EL layer formed of a first light-emitting unit 1103 a and a secondlight-emitting unit 1103 b between an anode 1101 and a cathode 1102.Further, an interlayer 1104 is provided between the first light-emittingunit 1103 a and the second light-emitting unit 1103 b.

When voltage higher than the threshold voltage of the light-emittingelement is applied between the anode 1101 and the cathode 1102, holesare injected to the EL layer from the anode 1101 side and electrons areinjected to the EL layer from the cathode 1102 side. The injectedelectrons and holes are recombined in the EL layer, so that alight-emitting substance contained in the EL layer emits light.

Note that in this specification, a layer or a stacked body whichincludes one region where electrons and holes injected from both endsare recombined is referred to as a light-emitting unit.

Note that the number of the light-emitting units provided between theanode 1101 and the cathode 1102 is not limited to two. A light-emittingelement exemplified in FIG. 7C has a structure in which a plurality oflight-emitting units 1103 is stacked, that is, a so-called tandemstructure. Note that in the case where n (n is a natural number greaterthan or equal to 2) light-emitting units 1103 are provided between theanode and the cathode, the interlayer 1104 is provided between an m-th(m is a natural number greater than or equal to 1 and less than or equalto n−1) light-emitting unit and an (m+1)-th light-emitting unit.

A light-emitting unit 1103 includes at least a light-emitting layercontaining a light-emitting substance, and may have a structure in whichthe light-emitting layer and a layer other than the light-emitting layerare stacked. Examples of the layer other than the light-emitting layerare layers containing a substance having a high hole-injection property,a substance having a high hole-transport property, a substance having apoor hole-transport property (substance which blocks holes), a substancehaving a high electron-transport property, a substance having a highelectron-injection property, and a substance having a bipolar property(substance having high electron- and hole-transport properties).

An example of a specific structure of the light-emitting unit 1103 isillustrated in FIG. 7B. In the light-emitting unit 1103 illustrated inFIG. 7B, a hole-injection layer 1113, a hole-transport layer 1114, alight-emitting layer 1115, an electron-transport layer 1116, and anelectron-injection layer 1117 are stacked in this order from the anode1101 side.

An example of a specific structure of the interlayer 1104 is illustratedin FIG. 7A. The interlayer 1104 may be formed to include at least acharge generation region, and may have a structure in which the chargegeneration region and a layer other than the charge generation regionare stacked. For example, a structure can be employed in which a firstcharge generation region 1104 c, an electron-relay layer 1104 b, and anelectron-injection buffer layer 1104 a are stacked in this order fromthe cathode 1102 side.

The behaviors of electrons and holes in the interlayer 1104 aredescribed. When a voltage higher than the threshold voltage of thelight-emitting element is applied between the anode 1101 and the cathode1102, in the first charge generation region 1104 c, holes and electronsare produced, and the holes move into the light-emitting unit 1103 b onthe cathode 1102 side and the electrons move into the electron-relaylayer 1104 b.

The electron-relay layer 1104 b has a high electron-transport propertyand immediately transfers the electrons generated in the first chargegeneration region 1104 c to the electron-injection buffer layer 1104 a.The electron-injection buffer layer 1104 a can reduce a barrier againstelectron injection into the light-emitting unit 1103, so that theefficiency of the electron injection into the light-emitting unit 1103can be improved. Thus, the electrons generated in the first chargegeneration region 1104 c are injected into the LUMO level of thelight-emitting unit 1103 through the electron-relay layer 1104 b and theelectron-injection buffer layer 1104 a.

In addition, the electron-relay layer 1104 b can prevent interaction inwhich the substance included in the first charge generation region 1104c and the substance included in the electron-injection buffer layer 1104a react with each other at the interface thereof and the functions ofthe first charge generation region 1104 c and the electron-injectionbuffer layer 1104 a are damaged.

The holes injected into the light-emitting unit 1103 b provided on thecathode side are recombined with the electrons injected from the cathode1102, so that a light-emitting substance contained in the light-emittingunit emits light. The electrons injected into the light-emitting unitprovided on the anode side are recombined with the holes injected fromthe anode side, so that a light-emitting substance contained in thelight-emitting unit emits light. Thus, the holes and electrons generatedin the interlayer 1104 cause light emission in the respectivelight-emitting units.

Note that the light-emitting units can be provided in contact with eachother when these light-emitting units allow the same structure as theinterlayer to be formed therebetween. Specifically, when one surface ofthe light-emitting unit is provided with a charge generation region, thecharge generation region functions as a first charge generation regionof the interlayer; thus, the light-emitting units can be provided incontact with each other.

Note that an interlayer can be provided between the cathode and the n-thlight-emitting unit.

<Material for Light-Emitting Element>

Next, specific materials that can be used for the light-emitting elementhaving the above-described structure are described. Materials for theanode, the cathode, the EL layer, the charge generation region, theelectron-relay layer, and the electron-injection buffer layer aredescribed in this order.

<Material for Anode>

The anode 1101 is preferably formed using a metal, an alloy, anelectrically conductive compound, a mixture of these materials, or thelike which has a high work function (specifically, a work function ofhigher than or equal to 4.0 eV is more preferable). Specifically, forexample, indium tin oxide (ITO), indium tin oxide containing silicon orsilicon oxide, indium zinc oxide (IZO), indium oxide containing tungstenoxide and zinc oxide, and the like are given.

Besides, as a material used for the anode 1101, the following can begiven: gold (Au), platinum (Pt), nickel (Ni), tungsten (W), chromium(Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper (Cu), palladium(Pd), titanium (Ti), nitride of a metal material (e.g., titaniumnitride), molybdenum oxide, vanadium oxide, ruthenium oxide, tungstenoxide, manganese oxide, titanium oxide, and the like.

Note that in the case where a second charge generation region isprovided in contact with the anode 1101, a variety of conductivematerials can be used for the anode 1101 regardless of their workfunctions. Specifically, besides a material which has a high workfunction, a material which has a low work function can also be used forthe anode 1101. A material for forming the second charge generationregion will be subsequently described together with a material forforming the first charge generation region.

<Material for Cathode>

As a material of the cathode 1102, a material having a low work function(specifically, a work function of lower than 4.0 eV) is preferably used;however, in the case where the first charge generation region isprovided between the cathode 1102 and the light-emitting unit 1103 to bein contact with the cathode 1102, various electrically conductivematerials can be used for the cathode 1102 regardless of their workfunctions.

Note that at least one of the cathode 1102 and the anode 1101 is formedusing a conductive film that transmits visible light. For the conductivefilm that transmits visible light, for example, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium tin oxide, indium zinc oxide, and indium tin oxide towhich silicon oxide is added can be given. Further, a metal thin filmhaving a thickness enough to transmit light (preferably, approximatelyfrom 5 nm to 30 nm) can also be used.

<Material for EL Layer>

Specific examples of materials for the layers included in thelight-emitting unit 1103 will be given below.

Hole-Injection Layer

The hole-injection layer is a layer having a high hole-injectionproperty. As the substance having a high hole-injection property, forexample, molybdenum oxide, vanadium oxide, ruthenium oxide, tungstenoxide, manganese oxide, or the like can be used. In addition, it ispossible to use a phthalocyanine-based compound such as phthalocyanine(H₂Pc) or copper phthalocyanine (CuPc), a high molecule such aspoly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS), orthe like to form the hole-injection layer.

Note that the second charge generation region may be used instead of thehole-injection layer. When the second charge generation region is used,a variety of conductive materials can be used for the anode 1101regardless of their work functions as described above. A material forforming the second charge generation region will be subsequentlydescribed together with a material for forming the first chargegeneration region.

Hole-Transport Layer

The hole-transport layer is a layer that contains a substance with ahigh hole-transport property. The hole-transport layer is not limited toa single layer, but may be a stacked-layer of two or more layers eachcontaining a substance having a high hole-transport property. Thehole-transport layer contains any substance having a higherhole-transport property than an electron-transport property, andpreferably contains a substance having a hole mobility of 10⁻⁶ cm²/Vs orhigher because the driving voltage of the light-emitting element can bereduced.

Light-Emitting Layer

The light-emitting layer contains a light-emitting substance. Thelight-emitting layer is not limited to a single layer, but may be astacked-layer of two or more layers containing light-emittingsubstances. As the light-emitting substance, a fluorescent compound or aphosphorescent compound can be used. A phosphorescent compound ispreferably used as the light-emitting substance because the emissionefficiency of the light-emitting element can be increased.

Those light-emitting substances are preferably dispersed in a hostmaterial. A host material preferably has higher excitation energy thanthe light-emitting substance.

Electron-Transport Layer

The electron-transport layer is a layer that contains a substance with ahigh electron-transport property. The electron-transport layer is notlimited to a single layer, but may be a stacked-layer of two or morelayers each containing a substance having a high electron-transportproperty. The electron-transport layer contains any substance having ahigher electron-transport property than a hole-transport property, andpreferably contains a substance having an electron mobility of 10⁻⁶cm²/Vs or higher because the driving voltage of the light-emittingelement can be reduced.

Electron-Injection Layer

The electron-injection layer is a layer including a highelectron-injection substance. The electron-injection layer is notlimited to a single layer, but may be a stacked-layer of two or morelayers containing substances having a high electron-injection property.The electron-injection layer is preferably provided because theefficiency of electron injection from the cathode 1102 can be increasedand the driving voltage of the light-emitting element can be reduced.

As the substance having a high electron-injection property, thefollowing can be given: an alkali metal and an alkaline earth metal suchas lithium (Li), cesium (Cs), calcium (Ca) and a compound thereof, suchas lithium fluoride (LiF), cesium fluoride (CsF), and calcium fluoride(CaF₂). Alternatively, a layer containing a substance having anelectron-transport property and an alkali metal, an alkaline earthmetal, magnesium (Mg), or a compound thereof (e.g., an Alq layercontaining magnesium (Mg)) can be used.

<Material for Charge Generation Region>

The first charge generation region 1104 c and the second chargegeneration region are regions containing a substance having a highhole-transport property and an acceptor substance. Note that the chargegeneration region is not limited to the structure in which one filmcontains the substance having a high hole-transport property and theacceptor substance, and may be a stacked layer of a layer containing thesubstance having a high hole-transport property and a layer containingthe acceptor substance. Note that in the case where the first chargegeneration region which is in contact with the cathode has astacked-layer structure, the layer containing the substance having ahigh hole-transport property is in contact with the cathode 1102. In thecase where the second charge generation region which is in contact withthe anode has a stacked-layer structure, the layer containing theacceptor substance is in contact with the anode 1101.

Note that the acceptor substance is preferably added to the chargegeneration region so that the mass ratio of the acceptor substance tothe substance having a high hole-transport property is from 0.1:1 to4.0:1.

As the acceptor substance that is used for the charge generation region,a transition metal oxide, particularly an oxide of a metal belonging toGroup 4 to 8 of the periodic table is preferred. Specifically,molybdenum oxide is particularly preferable. Note that molybdenum oxidehas a low hygroscopic property.

As the substance having a high hole-transport property used for thecharge generation region, any of a variety of organic compounds such asan aromatic amine compound, a carbazole derivative, an aromatichydrocarbon, and a high molecular compound (including an oligomer, adendrimer, or a polymer) can be used. Specifically, a substance having ahole mobility of 10⁻⁶ cm²/Vs or higher is preferably used. However,other substances than the above-described materials may also be used aslong as the substances have higher hole-transport properties thanelectron-transport properties.

<Material for Electron-Relay Layer>

The electron-relay layer 1104 b is a layer that can immediately receiveelectrons drawn out by the acceptor substance in the first chargegeneration region 1104 c. Therefore, the electron-relay layer 1104 b isa layer containing a substance having a high electron-transportproperty, and the LUMO level thereof is positioned between the acceptorlevel of the acceptor substance in the first charge generation region1104 c and the LUMO level of the light-emitting unit 1103. Specifically,the LUMO level is preferably about from −5.0 eV to −3.0 eV.

As the substance used for the electron-relay layer 1104 b, for example,a perylene derivative and a nitrogen-containing condensed aromaticcompound can be given. Note that a nitrogen-containing condensedaromatic compound is preferably used for the electron-relay layer 1104 bbecause of its stability. Among nitrogen-containing condensed aromaticcompounds, a compound having an electron-withdrawing group such as acyano group or fluorine is preferably used because such a compoundfurther facilitates acceptance of electrons in the electron-relay layer1104 b.

<Material for Electron-Injection Buffer Layer>

The electron-injection buffer layer 1104 a is a layer which facilitateselectron injection from the first charge generation region 1104 c intothe light-emitting unit 1103. By providing the electron-injection bufferlayer 1104 a between the first charge generation region 1104 c and thelight-emitting unit 1103, the injection barrier therebetween can bereduced.

A substance having a high electron-injection property can be used forthe electron-injection buffer layer 1104 a. For example, an alkalimetal, an alkaline earth metal, a rare earth metal, or a compoundthereof (e.g., an alkali metal compound (including an oxide such aslithium oxide, a halide, and carbonate such as lithium carbonate orcesium carbonate), an alkaline earth metal compound (including an oxide,a halide, and carbonate), or a rare earth metal compound (including anoxide, a halide, and carbonate)) can be used.

Further, in the case where the electron-injection buffer layer 1104 acontains a substance having a high electron-transport property and adonor substance with respect to the substance having a highelectron-transport property, the donor substance is preferably added sothat the mass ratio of the donor substance to the substance having ahigh electron-transport property is from 0.001:1 to 0.1:1. Note that asthe donor substance, an organic compound such as tetrathianaphthacene(abbreviation: TTN), nickelocene, or decamethylnickelocene can be usedas well as an alkali metal, an alkaline earth metal, a rare earth metal,a compound of the above metal (e.g., an alkali metal compound (includingan oxide of lithium oxide or the like, a halide, and carbonate such aslithium carbonate or cesium carbonate), an alkaline earth metal compound(including an oxide, a halide, and carbonate), and a rare earth metalcompound (including an oxide, a halide, and carbonate). Note that as thesubstance having a high electron-transport property, a material similarto the above-described material for the electron-transport layer whichcan be formed in part of the light-emitting unit 1103 can be used.

<Method of Manufacturing Light-Emitting Element>

A method for manufacturing the light-emitting element will be described.Over the first electrode, the layers described above are combined asappropriate to form an EL layer. Various methods (e.g., a dry process ora wet process) can be used to form the EL layer depending on thematerial for the EL layer. For example, a vacuum evaporation method, aninkjet method, a spin coating method, or the like may be selected. Notethat a different method may be employed for each layer. The secondelectrode is formed over the EL layer, so that the light-emittingelement is manufactured.

The light-emitting element described in this embodiment can befabricated by combination of the above-described materials. Lightemission from the above-described light-emitting substance can beobtained with this light-emitting element, and the emission color can beselected by changing the type of the light-emitting substance.

Further, a plurality of light-emitting substances which emit light ofdifferent colors can be used, whereby, for example, white light emissioncan also be obtained by expanding the width of the emission spectrum. Inorder to obtain white light emission, for example, a structure may beemployed in which at least two layers containing light-emittingsubstances are provided so that light of complementary colors isemitted. Specific examples of complementary colors include “blue andyellow”, “blue-green and red”, and the like.

Further, in order to obtain white light emission with an excellent colorrendering property, an emission spectrum preferably spreads through theentire visible light region. For example, a light-emitting element mayinclude layers emitting light exhibiting blue, green, and red.

This embodiment can be combined with any of the other embodimentsdisclosed in this specification as appropriate.

Embodiment 4

In this embodiment, electronic devices according to one embodiment ofthe present invention are described. Specifically, electronic deviceseach including a display device exemplified in the above embodiments aredescribed with reference to FIGS. 8A to 8E.

Examples of such an electronic device for which a display deviceaccording to one embodiment of the present invention is used include:television sets (also called TV or television receivers); monitors forcomputers or the like; cameras such as digital cameras or digital videocameras; digital photo frames; mobile phones (also called cellularphones or portable telephones); portable game machines; portableinformation terminals; audio playback devices; and large game machinessuch as pachinko machines. Specific examples of these electronic devicesare illustrated in FIG. 8A to 8E.

FIG. 8A illustrates an example of a television set. In a television set7100, a display portion 7103 is incorporated in a housing 7101. Imagescan be displayed on the display portion 7103. In addition, here, thehousing 7101 is supported by a stand 7105.

The television device 7100 can be operated by an operation switch of thehousing 7101 or a separate remote controller 7110. With operation keys7109 of the remote controller 7110, channels and volume can becontrolled and images displayed on the display portion 7103 can becontrolled. Furthermore, the remote controller 7110 may be provided witha display portion 7107 for displaying data output from the remotecontroller 7110.

Note that the television device 7100 is provided with a receiver, amodem, and the like. Moreover, when the display device is connected to acommunication network with or without wires via the modem, one-way (froma sender to a receiver) or two-way (between a sender and a receiver orbetween receivers) information communication can be performed.

FIG. 8B illustrates a computer, which includes a main body 7201, ahousing 7202, a display portion 7203, a keyboard 7204, an externalconnecting port 7205, a pointing device 7206, and the like. Note thatthe display device of one embodiment of the present invention is appliedto the display portion 7203 in this computer.

FIG. 8C illustrates a portable game machine, which includes twohousings, a housing 7301 and a housing 7302, which are connected with ajoint portion 7303 so that the portable game machine can be opened orfolded. A display portion 7304 is incorporated in the housing 7301 and adisplay portion 7305 is incorporated in the housing 7302. The portablegame console in FIG. 8C also includes a speaker portion 7306, arecording medium insertion portion 7307, an LED lamp 7308, input means(an operation key 7309, a connection terminal 7310, a sensor 7311 (asensor having a function of measuring force, displacement, position,speed, acceleration, angular velocity, rotational frequency, distance,light, liquid, magnetism, temperature, chemical substance, sound, time,hardness, electric field, current, voltage, electric power, radiation,flow rate, humidity, tilt angle, vibration, smell, or infrared rays),and a microphone 7312), and the like. It is needless to say that thestructure of the portable game machine is not limited to the above aslong as the display device of one embodiment of the present invention isused for at least either the display portion 7304 or the display portion7305, or both, and can include other accessories arbitrarily. Theportable game console in FIG. 8C has a function of reading a program ordata stored in a recording medium to display it on the display portion,and a function of sharing information with another portable game consoleby wireless communication. The portable game machine in FIG. 8C can havea variety of functions without limitation to the above functions.

FIG. 8D illustrates an example of a cellular phone. The cellular phone7400 is provided with a display portion 7402 incorporated in a housing7401, operation buttons 7403, an external connection port 7404, aspeaker 7405, a microphone 7406, and the like. Note that the cellularphone 7400 is manufactured by using the display device of one embodimentof the present invention for the display portion 7402.

When the display portion 7402 of the cellular phone 7400 illustrated inFIG. 8D is touched with a finger or the like, data can be input into thecellular phone 7400. Further, operations such as making a call andcreating e-mail can be performed by touch on the display portion 7402with a finger or the like.

There are mainly three screen modes of the display portion 7402. Thefirst mode is a display mode mainly for displaying images. The secondmode is an input mode mainly for inputting data such as text. The thirdmode is a display-and-input mode in which two modes of the display modeand the input mode are combined.

For example, in the case of making a call or composing an e-mail, a textinput mode mainly for inputting text is selected for the display portion7402 so that text displayed on a screen can be inputted. In this case,it is preferable to display a keyboard or number buttons on almost allthe area of the screen of the display portion 7402.

When a detection device including a sensor for detecting inclination,such as a gyroscope or an acceleration sensor, is provided inside thecellular phone 7400, display on the screen of the display portion 7402can be automatically changed by determining the orientation of thecellular phone 7400 (whether the cellular phone is placed horizontallyor vertically for a landscape mode or a portrait mode).

The screen modes are switched by touching the display portion 7402 oroperating the operation buttons 7403 of the housing 7401. Alternatively,the screen modes can be switched depending on kinds of images displayedon the display portion 7402. For example, when a signal of an imagedisplayed on the display portion is a signal of moving image data, thescreen mode is switched to the display mode. When the signal is a signalof text data, the screen mode is switched to the input mode.

Moreover, in the input mode, when input by touching the display portion7402 is not performed within a specified period while a signal detectedby an optical sensor in the display portion 7402 is detected, the screenmode may be controlled so as to be switched from the input mode to thedisplay mode.

The display portion 7402 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken by touchon the display portion 7402 with the palm or the finger, wherebypersonal authentication can be performed. Further, by providing abacklight or a sensing light source which emits a near-infrared light inthe display portion, an image of a finger vein, a palm vein, or the likecan be taken.

FIG. 8E illustrates an example of a folding computer. A folding computer7450 includes a housing 7451L and a housing 7451R connected by hinges7454. The folding computer 7450 further includes an operation button7453, a left speaker 7455L, and a right speaker 7455R. In addition, aside surface of the folding computer 7450 is provided with an externalconnection port 7456, which is not illustrated. When the hinge 7454 isfolded so that a display portion 7452L provided in the housing 7451L anda display portion 7452R provided in the housing 7451R face each other,the display portions can be protected by the housings. The foldingcomputer 7450 includes the display device of one embodiment of thepresent invention at least either the display portion 7452L or thedisplay portion 7452R, or both.

Each of the display portions 7452L and 7452R is a component which candisplay images and to which information can be input by touch with afinger or the like. For example, the icon for the installed program isselected by touch with a finger, so that the program can be started.Further, changing the distance between fingers touching two positions ofthe displayed image enables zooming in or out on the image. Drag of afinger touching one position of the displayed image enables drag anddrop of the image. Selection of the displayed character or symbol on thedisplayed image of a keyboard by touch with a finger enables informationinput.

Further, the computer 7450 can also include a gyroscope, an accelerationsensor, a global positioning system (GPS) receiver, fingerprint sensor,or a video camera. For example, a detection device including a sensorwhich detects inclination, such as a gyroscope or an accelerationsensor, is provided to determine the orientation of the computer 7450(whether the computer is placed horizontally or vertically for alandscape mode or a portrait mode) so that the orientation of thedisplay screen can be automatically changed.

Furthermore, the computer 7450 can be connected to a network. Thecomputer 7450 not only can display information on the Internet but alsocan be used as a terminal which controls another electronic deviceconnected to the network from a distant place.

The display device of one embodiment of the present invention includes adisplay panel in a thin film form. Thus, when the display device isattached to a base with a curved surface, the display device with acurved surface can be obtained. In addition, when the display device islocated in a housing with a curved surface, an electronic device with acurved surface can be obtained.

This embodiment can be combined with any of the other embodimentsdisclosed in this specification as appropriate.

Example

A light-emitting module for emitting light exhibiting blue color, alight-emitting module for emitting light exhibiting green color, and alight-emitting module for emitting light exhibiting red color, which areapplicable to a display device which is one embodiment of the presentinvention, were manufactured and the characteristics thereof weremeasured. The results of the measurement are described below.

<Structure of Light-Emitting Element Manufactured>

FIG. 9 shows a structure of a light-emitting element used for thelight-emitting modules manufactured. In each of the light-emittingmodules manufactured, a first electrode 251 also served as a reflectivefilm, a second electrode 252 also served as a semi-transmissive andsemi-reflective film, and a layer 253 containing a light-emittingorganic compound was provided between the first electrode and the secondelectrode.

<<Structure of First Electrode>>

In each of the light-emitting modules, the first electrode 251 alsoserving as a reflective film was formed using a stack in which a6-nm-thick titanium film was formed over a 200-nm-thickaluminum-titanium alloy film. As an optical adjustment layer over thereflective film, an indium tin oxide film containing silicon oxide(abbreviation: ITSO) was used. The thickness of the optical adjustmentlayer was optimized for light emission color.

Specifically, the light-emitting module for emitting light exhibitinggreen color was provided with a 40-nm-thick ITSO film as the opticaladjustment layer, and the light-emitting module for emitting lightexhibiting red color was provided with an 80-nm-thick ITSO film as theoptical adjustment layer. Note that the light-emitting module foremitting light exhibiting blue color was provided with a layercontaining a light-emitting organic compound being in contact with the6-nm-thick titanium film.

<<Structure of Second Electrode>>

As the second electrode 252, a conductive film in which 70-nm-thickindium tin oxide (abbreviation: ITO) was stacked on a 15-nm-thicksilver-magnesium alloy film was used. The silver-magnesium alloy filmwas fainted by co-evaporation with the weight ratio of 10:1 (=Ag:Mg).

<<Structure of Layer Containing Light-Emitting Organic Compound>>

As illustrated in FIG. 9, a layer 253 containing a light-emittingorganic compound had a structure in which two EL layers (a first ELlayer 1503 a and a second EL layer 1503 b) were provided with aninterlayer 1504 interposed therebetween (the structure is also referredto as tandem structure).

The first EL layer 1503 a included a hole-injection layer 1511, a firsthole-transport layer 1512, a first light-emitting layer 1513, a firstelectron-transport layer 1514 a, and a second electron-transport layer1514 b in this order over the first electrode 251.

The interlayer 1504 included an electron-injection buffer layer 1504 a,an electron-relay layer 1504 b, and a charge-generation region 1504 c inthis order over the electron-transport layer 1514 b.

The second EL layer 1503 b included a second hole-transport layer 1522,a second light-emitting layer 1523 a, a third light-emitting layer 1523b, a third electron-transport layer 1524 a, a fourth electron-transportlayer 1524 b, and an electron-injection layer 1525 in this order overthe interlayer 1504.

Table 1 shows details of materials included in the layer containing alight-emitting organic compound.

TABLE 1 First EL layer 1503a Interlayer 1504 Light- Electron- Hole-Hole- emitting injection Electron- Charge- injection transport layerFirst Electron-transport layer buffer layer relay generation layer 1511layer 1512 1513 1514a 1514b 1504a layer 1504b region 1504c EL layerPCzPA:MoOx PCzPA CzPA:1,6- CzPA Bphen Li CuPc BPAFLP:MoOx (=2:1) 20 nmmMemFLPAPrn 5 nm 15 nm 0.1 nm 2 nm (=2:1) 10 nm (=1:0.05) 15 nm 30 nmSecond EL layer 1503b Hole- Light-emitting layer Electron-transportlayer Electron- transport Second Third Third Fourth injection layer 15221523a 1523b 1524a 1524b layer 1525 EL layer BPAFLP2mDBTPDBqII:PCBA1BP:Ir(tBppm)2acac 2mDBTPDBqII:Ir(tppr)2dpm 2mDBTPDBqIIBphen LiF 20 nm 0.8:0.2:0.06 1:0.02 15 nm 15 nm 1 nm 20 nm 20 nm

Shown below are structural formulae of some of the organic compoundsused in this example.

<Evaluation Results>

The results of measuring the characteristics of the light-emittingmodules manufactured are described below.

<<Emission Spectrum>>

FIGS. 10A to 10C each show the result of measuring the spectrum of lightemitted from the light-emitting module manufactured.

FIG. 10A is an emission spectrum of the light-emitting module foremitting light exhibiting red color. The half width of the spectrum was41 nm.

FIG. 10B is an emission spectrum of the light-emitting module foremitting light exhibiting green color. The half width of the spectrumwas 31 nm.

FIG. 10C is an emission spectrum of the light-emitting module foremitting light exhibiting blue color. The half width of the spectrum was23 nm.

The half width of the spectrum of light emitted from each light-emittingmodule was narrower than 50 nm, and the light exhibited bright color.

<<Color Reproduction Characteristics>>

FIG. 11 is a chromaticity diagram in which colors of light emitted fromthe above three light-emitting modules are plotted. A dashed-linetriangle whose vertexes correspond to the plotted points represents arange of color which can be displayed using the light-emitting modules.The proportion of the area of the dashed-line triangle in the area of asolid-line triangle whose vertexes are based on National TelevisionSystem Committee (NTSC) standard was 95.4%.

<<Response Time>>

FIG. 12A shows luminance of light emitted from the light-emittingmodule, with respect to time elapsed from emission start. FIG. 12B isobtained by increasing the scale of the horizontal axis of FIG. 12A.Note that the luminance on the vertical axis represents luminancenormalized under the condition that luminance at the time when luminanceof light emitted from the light-emitting module becomes stable isregarded as 100%.

Among two kinds of curve in FIGS. 12A and 12B, Sample 1 represents themeasurement result of the light-emitting module which emits lightexhibiting blue color, and Sample 2 represents the measurement result ofthe light-emitting module which emits light exhibiting green color.

Response time refers to time from the point when emission started to thepoint when luminance reached 90%. The response time of Sample 1 wasapproximately 7 μs, and the response time of Sample 2 was approximately24 μs. Thus, it can be confirmed that each of the light-emitting moduleshas a very short response time.

Note that this example can be combined with other embodiments in thisspecification as appropriate.

EXPLANATION OF REFERENCE

100: display device, 101: display portion, 103: driver circuit portion,104: driver circuit portion, 105: control circuit portion, 107: imageprocessing device, 109: decoder circuit portion, 111: grayscaleconversion portion, 113: memory portion, 114: noise removing portion,115: pixel-to-pixel complementary portion, 116: tone correction portion,117: complementary frame generating portion, 251: electrode, 252:electrode, 253: layer, 400: display panel, 402: pixel, 402B: sub pixel,402G: sub pixel, 402R: sub pixel, 405: sealant, 408: wiring, 410:substrate, 411: transistor, 412: transistor, 413: n-channel transistor,414: p-channel transistor, 416: insulating layer, 418: partition, 420B:light-emitting element, 420G: light-emitting element, 420R:light-emitting element, 421B: electrode, 421G: electrode, 421R:electrode, 422: electrode, 423: layer, 423 a: layer, 423 b: layer, 424:interlayer, 431: space, 440: substrate, 441B: color filter, 441G: colorfilter, 442: film, 450B: light-emitting module, 450G: light-emittingmodule, 450R: light-emitting module, 1101: anode, 1102: cathode, 1103:light-emitting unit, 1103 a: light-emitting unit, 1103 b: light-emittingunit, 1104: interlayer, 1104 a: electron-injection buffer layer, 1104 b:electron-relay layer, 1104 c: charge-generation region, 1113:hole-injection layer, 1114: hole-transport layer, 1115: light-emittinglayer, 1116: electron-transport layer, 1117: electron-injection layer,1503 a: EL layer, 1503 b: EL layer, 1504: interlayer, 1504 a:electron-injection buffer layer, 1504 b: electron-relay layer, 1504 c:charge-generation region, 1511: hole-injection layer, 1512:hole-transport layer, 1513: light-emitting layer, 1514 a:electron-transport layer, 1514 b: electron-transport layer, 1522:hole-transport layer, 1523 a: light-emitting layer, 1523 b:light-emitting layer, 1524 a: electron-transport layer, 1524 b:electron-transport layer, 1525: electron-injection layer, 7100:television set, 7101: housing, 7103: display portion, 7105: stand, 7107:display portion, 7109: operation key, 7110: separate remote controller,7201: main body, 7202: housing, 7203: display portion, 7204: keyboard,7205: external connecting port, 7206: pointing device, 7301: housing,7302: housing, 7303: connection portion, 7304: display portion, 7305:display portion, 7306: speaker portion, 7307: recording medium insertionportion, 7308: LED lamp, 7309: operation key, 7310: connection terminal,7311: sensor, 7312: microphone, 7400: cellular phone, 7401: housing,7402: display portion, 7403: operation button, 7404: external connectionport, 7405: speaker, 7406: microphone, 7450: computer, 7451L: housing,7451R: housing, 7452L: display portion, 7452R: display portion, 7453:operation button, 7454: hinge, 7455L: left speaker, 7455R: rightspeaker, 7456: external connection port.

This application is based on Japanese Patent Application serial No.2012-087818 filed with Japan Patent Office on Apr. 6, 2012, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A display device comprising: a grayscaleconversion portion; and a display portion, wherein the grayscaleconversion portion is configured to convert a first image signalinputted to the grayscale conversion portion into a second image signaland transmit the second image signal to the display portion, wherein thefirst image signal has p gray levels (p is a natural number and an evennumber), wherein the second image signal is subjected to a grayscaleconversion processing in which each of q gray levels from the lowestgray level to the q-th gray level is further divided into 2^(r) graylevels, and wherein q is greater than or equal to 1 and less than orequal to p/2 and r is greater than or equal to 1 and less than or equalto
 20. 2. The display device according to claim 1, wherein the grayscaleconversion portion is configured to divide an image contained in thefirst image signal into a plurality of regions, extract luminancedistribution in each of the plurality of regions, determine whether toperform the grayscale conversion processing in each of the plurality ofregions based on the luminance distribution, and perform the grayscaleconversion processing to a region which needs the grayscale conversionprocessing.
 3. The display device according to claim 1, wherein thedisplay portion comprises a pixel, wherein the pixel comprises one of afirst light-emitting module, a second light-emitting module, and a thirdlight-emitting module, wherein the first light-emitting modulecomprises: a first reflective film; a first light-emitting layer overthe first reflective film; an interlayer over the first light-emittinglayer; a second light-emitting layer over the interlayer; asemi-transmissive and semi-reflective film over the secondlight-emitting layer; and a first color filter for transmitting lightexhibiting red color over the semi-transmissive and semi-reflectivefilm, wherein a first optical path length between the first reflectivefilm and the semi-transmissive and semi-reflective film is adjusted toi/2 times (i is a natural number) the length greater than or equal to600 nm and less than 800 nm, wherein the second light-emitting modulecomprises: a second reflective film; the first light-emitting layer overthe second reflective film; the interlayer over the first light-emittinglayer; the second light-emitting layer over the interlayer; thesemi-transmissive and semi-reflective film over the secondlight-emitting layer; and a second color filter for transmitting lightexhibiting green color over the semi-transmissive and semi-reflectivefilm, wherein a second optical path length between the second reflectivefilm and the semi-transmissive and semi-reflective film is adjusted toj/2 times (j is a natural number) the length greater than or equal to500 nm and less than 600 nm, and wherein the third light-emitting modulecomprises: a third reflective film; the first light-emitting layer overthe third reflective film; the interlayer over the first light-emittinglayer; the second light-emitting layer over the interlayer; thesemi-transmissive and semi-reflective film over the secondlight-emitting layer; and a third color filter for transmitting lightexhibiting blue color over the semi-transmissive and semi-reflectivefilm, wherein a third optical length between the third reflective filmand the semi-transmissive and semi-reflective film is adjusted to k/2times (k is a natural number) the length greater than or equal to 400 nmand less than 500 nm.
 4. The display device according to claim 1,wherein the display portion comprises a pixel, wherein the pixelcomprises one of a first light-emitting module, a second light-emittingmodule, and a third light-emitting module, wherein the firstlight-emitting module comprises: a first reflective film; a firstelectrode over the first reflective film; a first light-emitting layerover the first electrode; an interlayer over the first light-emittinglayer; a second light-emitting layer over the interlayer; asemi-transmissive and semi-reflective film over the secondlight-emitting layer; and a first color filter for transmitting lightexhibiting red color over the semi-transmissive and semi-reflectivefilm, wherein the second light-emitting module comprises: a secondreflective film; a second electrode over the second reflective film; thefirst light-emitting layer over the second electrode; the interlayerover the first light-emitting layer; the second light-emitting layerover the interlayer; the semi-transmissive and semi-reflective film overthe second light-emitting layer; and a second color filter fortransmitting light exhibiting green color over the semi-transmissive andsemi-reflective film, and wherein the third light-emitting modulecomprises: a third reflective film; a third electrode over the thirdreflective film; the first light-emitting layer over the thirdelectrode; the interlayer over the first light-emitting layer; thesecond light-emitting layer over the interlayer; the semi-transmissiveand semi-reflective film over the second light-emitting layer; and athird color filter for transmitting light exhibiting blue color over thesemi-transmissive and semi-reflective film.
 5. The display deviceaccording to claim 1, wherein the display portion comprises a pixelcomprising a light-emitting module, and wherein the light-emittingmodule comprises: a reflective film; a first light-emitting layer overthe reflective film; an interlayer over the first light-emitting layer;a second light-emitting layer over the interlayer; a semi-transmissiveand semi-reflective film over the second light-emitting layer; and acolor filter for transmitting light exhibiting red color over thesemi-transmissive and semi-reflective film, wherein the light-emittingmodule is configured to emit light that exhibits red and has a spectralline having a peak in which a half width of the peak is less than 50 nm.6. An electronic device comprising the display device according toclaim
 1. 7. A display device comprising: a display portion comprising aplurality of pixels; one of the plurality of pixels comprising: alight-emitting module capable of emitting light with a spectral linehaving a peak in which a half width of the peak is 60 nm or less withina response time of 100 μs; and a grayscale conversion portion, whereinthe display portion has an NTSC ratio of 80% or higher and a contrastratio of 500 or higher, wherein the plurality of pixels are provided ata resolution of 80 ppi or more, wherein the grayscale conversion portionis configured to convert a first image signal inputted to the grayscaleconversion portion into a second image signal and transmit the secondimage signal to the display portion, wherein the first image signal hasp gray levels (p is a natural number and an even number), wherein thesecond image signal is subjected to a grayscale conversion processing inwhich each of q gray levels from the lowest gray level to the q-th graylevel is further divided into 2^(r) gray levels, and wherein q isgreater than or equal to 1 and less than or equal to p/2 and r isgreater than or equal to 1 and less than or equal to
 20. 8. The displaydevice according to claim 7, wherein the grayscale conversion portion isconfigured to divide an image contained in the first image signal into aplurality of regions, extract luminance distribution in each of theplurality of regions, determine whether to perform the grayscaleconversion processing in each of the plurality of regions based on theluminance distribution, and perform the grayscale conversion processingto a region which needs the grayscale conversion processing.
 9. Thedisplay device according to claim 7, wherein the light-emitting moduleis one of a first light-emitting module, a second light-emitting module,and a third light-emitting module, wherein the first light-emittingmodule comprises: a first reflective film; a first light-emitting layerover the first reflective film; an interlayer over the firstlight-emitting layer; a second light-emitting layer over the interlayer;a semi-transmissive and semi-reflective film over the secondlight-emitting layer; and a first color filter for transmitting lightexhibiting red color over the semi-transmissive and semi-reflectivefilm, wherein a first optical path length between the first reflectivefilm and the semi-transmissive and semi-reflective film is adjusted toi/2 times (i is a natural number) the length greater than or equal to600 nm and less than 800 nm, wherein the second light-emitting modulecomprises: a second reflective film; the first light-emitting layer overthe second reflective film; the interlayer over the first light-emittinglayer; the second light-emitting layer over the interlayer; thesemi-transmissive and semi-reflective film over the secondlight-emitting layer; and a second color filter for transmitting lightexhibiting green color over the semi-transmissive and semi-reflectivefilm, wherein a second optical path length between the second reflectivefilm and the semi-transmissive and semi-reflective film is adjusted toj/2 times (j is a natural number) the length greater than or equal to500 nm and less than 600 nm, and wherein the third light-emitting modulecomprises: a third reflective film; the first light-emitting layer overthe third reflective film; the interlayer over the first light-emittinglayer; the second light-emitting layer over the interlayer; thesemi-transmissive and semi-reflective film over the secondlight-emitting layer; and a third color filter for transmitting lightexhibiting blue color over the semi-transmissive and semi-reflectivefilm, wherein a third optical length between the third reflective filmand the semi-transmissive and semi-reflective film is adjusted to k/2times (k is a natural number) the length greater than or equal to 400 nmand less than 500 nm.
 10. The display device according to claim 7,wherein the light-emitting module is one of a first light-emittingmodule, a second light-emitting module, and a third light-emittingmodule, wherein the first light-emitting module comprises: a firstreflective film; a first electrode over the first reflective film; afirst light-emitting layer over the first electrode; an interlayer overthe first light-emitting layer; a second light-emitting layer over theinterlayer; a semi-transmissive and semi-reflective film over the secondlight-emitting layer; and a first color filter for transmitting lightexhibiting red color over the semi-transmissive and semi-reflectivefilm, wherein the second light-emitting module comprises: a secondreflective film; a second electrode over the second reflective film; thefirst light-emitting layer over the second electrode; the interlayerover the first light-emitting layer; the second light-emitting layerover the interlayer; the semi-transmissive and semi-reflective film overthe second light-emitting layer; and a second color filter fortransmitting light exhibiting green color over the semi-transmissive andsemi-reflective film, and wherein the third light-emitting modulecomprises: a third reflective film; a third electrode over the thirdreflective film; the first light-emitting layer over the thirdelectrode; the interlayer over the first light-emitting layer; thesecond light-emitting layer over the interlayer; the semi-transmissiveand semi-reflective film over the second light-emitting layer; and athird color filter for transmitting light exhibiting blue color over thesemi-transmissive and semi-reflective film.
 11. The display deviceaccording to claim 7, wherein the light-emitting module comprises: areflective film; a first light-emitting layer over the reflective film;an interlayer over the first light-emitting layer; a secondlight-emitting layer over the interlayer; a semi-transmissive andsemi-reflective film over the second light-emitting layer; and a colorfilter for transmitting light exhibiting red color over thesemi-transmissive and semi-reflective film, wherein the light-emittingmodule is configured to emit light that exhibits red and has a spectralline having a peak in which a half width of the peak is less than 50 nm.12. An electronic device comprising the display device according toclaim
 7. 13. A display device comprising: a grayscale conversionportion; and a display portion comprising a plurality of pixels, whereinone of the plurality of pixels comprising a first light-emitting module,a second light-emitting module, and a third light-emitting module whichare capable of emitting light with a spectral line having a peak inwhich a half width of the peak is 60 nm or less within a response timeof 100 μs, wherein the display portion has an NTSC ratio of 80% orhigher and a contrast ratio of 500 or higher, wherein the plurality ofpixels are provided at a resolution of 80 ppi or more, wherein thegrayscale conversion portion is configured to convert a first imagesignal inputted to the grayscale conversion portion into a second imagesignal and transmit the second image signal to the display portion,wherein the first image signal has p gray levels (p is a natural numberand an even number), wherein the second image signal is subjected to agrayscale conversion processing in which each of q gray levels from thelowest gray level to the q-th gray level is further divided into 2^(r)gray levels, and wherein q is greater than or equal to 1 and less thanor equal to p/2 and r is greater than or equal to 1 and less than orequal to
 20. 14. The display device according to claim 13, wherein thegrayscale conversion portion is configured to divide an image containedin the first image signal into a plurality of regions, extract luminancedistribution in each of the plurality of regions, determine whether toperform the grayscale conversion processing in each of the plurality ofregions based on the luminance distribution, and perform the grayscaleconversion processing to a region which needs the grayscale conversionprocessing.
 15. The display device according to claim 13, wherein thefirst light-emitting module comprises: a first reflective film; a firstlight-emitting layer over the first reflective film; an interlayer overthe first light-emitting layer; a second light-emitting layer over theinterlayer; a semi-transmissive and semi-reflective film over the secondlight-emitting layer; and a first color filter for transmitting lightexhibiting red color over the semi-transmissive and semi-reflectivefilm, wherein a first optical path length between the first reflectivefilm and the semi-transmissive and semi-reflective film is adjusted toi/2 times (i is a natural number) the length greater than or equal to600 nm and less than 800 nm, wherein the second light-emitting modulecomprises: a second reflective film; the first light-emitting layer overthe second reflective film; the interlayer over the first light-emittinglayer; the second light-emitting layer over the interlayer; thesemi-transmissive and semi-reflective film over the secondlight-emitting layer; and a second color filter for transmitting lightexhibiting green color over the semi-transmissive and semi-reflectivefilm, wherein a second optical path length between the second reflectivefilm and the semi-transmissive and semi-reflective film is adjusted toj/2 times (j is a natural number) the length greater than or equal to500 nm and less than 600 nm, and wherein the third light-emitting modulecomprises: a third reflective film; the first light-emitting layer overthe third reflective film; the interlayer over the first light-emittinglayer; the second light-emitting layer over the interlayer; thesemi-transmissive and semi-reflective film over the secondlight-emitting layer; and a third color filter for transmitting lightexhibiting blue color over the semi-transmissive and semi-reflectivefilm, wherein a third optical length between the third reflective filmand the semi-transmissive and semi-reflective film is adjusted to k/2times (k is a natural number) the length greater than or equal to 400 nmand less than 500 nm.
 16. The display device according to claim 13,wherein the first light-emitting module comprises: a first reflectivefilm; a first electrode over the first reflective film; a firstlight-emitting layer over the first electrode; an interlayer over thefirst light-emitting layer; a second light-emitting layer over theinterlayer; a semi-transmissive and semi-reflective film over the secondlight-emitting layer; and a first color filter for transmitting lightexhibiting red color over the semi-transmissive and semi-reflectivefilm, wherein the second light-emitting module comprises: a secondreflective film; a second electrode over the second reflective film; thefirst light-emitting layer over the second electrode; the interlayerover the first light-emitting layer; the second light-emitting layerover the interlayer; the semi-transmissive and semi-reflective film overthe second light-emitting layer; and a second color filter fortransmitting light exhibiting green color over the semi-transmissive andsemi-reflective film, and wherein the third light-emitting modulecomprises: a third reflective film; a third electrode over the thirdreflective film; the first light-emitting layer over the thirdelectrode; the interlayer over the first light-emitting layer; thesecond light-emitting layer over the interlayer; the semi-transmissiveand semi-reflective film over the second light-emitting layer; and athird color filter for transmitting light exhibiting blue color over thesemi-transmissive and semi-reflective film.
 17. The display deviceaccording to claim 16, wherein the first light-emitting module isconfigured to emit light that exhibits red and has a first spectral linehaving a first peak in which a first half width of the first peak isless than 50 nm, wherein the second light-emitting module is configuredto emit light that exhibits green and has a second spectral line havinga second peak in which a second half width of the second peak isnarrower than the first half width, and wherein the third light-emittingmodule is configured to emit light that exhibits blue and has a thirdspectral line having a third peak in which a third half width of thethird peak is narrower than the second half width.
 18. An electronicdevice comprising the display device according to claim 13.